A conventional example of an intake manifold is provided upstream of a combustion chamber of an engine. A gas mixture is introduced into the combustion chamber through the intake manifold. Since EGR gas or blow-by gas is introduced through the intake manifold, oil or water contained in the EGR gas or the blow-by gas is accumulated within the intake manifold. If a large volume of oil is introduced into the engine, a knocking may occur due to incomplete combustion. Also, a large volume of water introduced into the engine could be a cause of misfire. Patent Document 1 identified below relates to a technique for discharging oil or water accumulated in the intake manifold properly.
A construction for discharging oil accumulated in an intake passage according to Patent Document 1 includes, in addition to the intake passage, an intake throttle valve and a communicating pipe. Air discharged from an exhaust port undergoes predetermined processing in the intake passage to be supplied to an intake port. The intake throttle valve is provided within the intake passage and is configured to open the intake passage full when the drive of an internal combustion engine is stopped. The communicating pipe is arranged parallel with the intake passage between an oil reservoir of the intake passage and the intake throttle valve to allow the oil reservoir to communicate with the vicinity of the intake throttle valve adjacent the internal combustion engine. This system causes oil accumulated in the intake passage to be drawn up by negative pressure and returned to the intake passage.